1. Field of the Invention
This invention relates to electronic circuits used for driving a motor used to actuate a Heating-Ventilation-Air Conditioning (HVAC) blend door in an automobile.
2. Description of the Relevant Art
The problem addressed by this invention is encountered in the automotive industry where the temperature of the passenger area of an automobile is controlled by moving a temperature control, typically located on the dashboard. By moving the temperature control, the blend of fresh air and hot air is modified. The temperature control is typically attached to a potentiometer which generates a control voltage proportional to the position of the temperature control. The control voltage can also be generated by an automotive temperature control module.
FIG. 1 is a schematic drawing of a prior art circuit for controlling the position of an HVAC blend door responsive to the control voltage through resistor 8. The HVAC blend door is attached to the shaft motor 24, but is not shown in this drawing. In this circuit for controlling the position of and HVAC blend door, an operational amplifier 20 and 28 control the position of motor 24. The motor 24 also has a potentiometer 6 attached to the shaft of the motor such that the rotation of the motor results in the rotation of the potentiometer 6 as well as the blend door. Thus, the motor is positioned by changing the control voltage which is sensed on the inverting input of amplifier 20. The output of amplifier 20 responds to the control voltage in either a high or a low voltage. Because of the interconnection through resistor 22, amplifier 28 responds to the control voltage with an opposite output voltage as amplifier 20. Thus, motor 24 is rotated in a first direction when the control voltage is increased and is rotated in a second direction when the control voltage is decreased.
As motor 24 rotates, the blend door and the potentiometer rotates. Consequently, the voltage on the sweeper of the potentiometer approaches the control voltage which causes the outputs of amplifiers 20 and 28 to return to their static voltages until the temperature control is moved again.
As discussed above, the circuit is used in automobile where the circuit is exposed to harsh temperatures and to severe shock and vibration. Additionally, the automotive industry is very cost sensitive since the success of a particular automobile is very price sensitive.